package com.thealgorithms.maths;

import java.math.BigInteger;
import java.util.*;

public class KaprekarNumbers {

    /* This program demonstrates if a given number is Kaprekar Number or not.
	Kaprekar Number: A Kaprekar number is an n-digit number which its square can be split into two parts where the right part has n
	digits and sum of these parts is equal to the original number. */

    // Provides a list of kaprekarNumber in a range
    public static List<Long> kaprekarNumberInRange(long start, long end)
        throws Exception {
        long n = end - start;
        if (n < 0) throw new Exception("Invalid range");
        ArrayList<Long> list = new ArrayList<>();

        for (long i = start; i <= end; i++) {
            if (isKaprekarNumber(i)) list.add(i);
        }

        return list;
    }

    // Checks whether a given number is Kaprekar Number or not
    public static boolean isKaprekarNumber(long num) {
        String number = Long.toString(num);
        BigInteger originalNumber = BigInteger.valueOf(num);
        BigInteger numberSquared = originalNumber.multiply(originalNumber);
        if (number.length() == numberSquared.toString().length()) {
            return number.equals(numberSquared.toString());
        } else {
            BigInteger leftDigits1 = BigInteger.ZERO;
            BigInteger leftDigits2;
            if (numberSquared.toString().contains("0")) {
                leftDigits1 =
                    new BigInteger(
                        numberSquared
                            .toString()
                            .substring(0, numberSquared.toString().indexOf("0"))
                    );
            }
            leftDigits2 =
                new BigInteger(
                    numberSquared
                        .toString()
                        .substring(
                            0,
                            (
                                numberSquared.toString().length() -
                                number.length()
                            )
                        )
                );
            BigInteger rightDigits = new BigInteger(
                numberSquared
                    .toString()
                    .substring(
                        numberSquared.toString().length() - number.length()
                    )
            );
            String x = leftDigits1.add(rightDigits).toString();
            String y = leftDigits2.add(rightDigits).toString();
            return (number.equals(x)) || (number.equals(y));
        }
    }
}
